This is actually a classic algorithm used for things like dealing a pack of cards. If you are dealing bridge hands, for example, you want 52 different cards with no repeats. Some games use multiple packs and/or cut down packs (Bézique uses a pack of 2 decks, A down to 7). You therefore set up a pack with all the possible cards, deal one at random, move the last card to the position from which the card was taken & reduce the pack by one. So, for your problem:

use strict;
use warnings;

my $nRepeats = 2;
my @sSet = qw (a b c d e f);
my $nLength = 12;

my @sAll;
for (1..$nRepeats) {
    push (@sAll, @sSet);
}

if ($nLength - 1 > $#sAll) {die "Don't be silly"}

my $sString;
for (1..$nLength) {
    my $i = int(rand(@sAll));
    $sString .= $sAll[$i];
    $sAll[$i] = $sAll[-1];
    pop(@sAll);
}

print "$sString \n";
[download]

Now, I'm a beginner. So if anyone sees anything that could be improved (apart from Hungarian notation - it helps me conceptualise strong typing), I'd love to know.

Regards,

John Davies

Update: Re-reading your question, if AABAABAABAAB is valid, my code won't do what you want, but I'm not clear on whether that's what you mean.